As methods for synthesizing hydrocarbons from the synthesis gas, a Fischer-Tropsch reaction (hereinafter referred to as the “FT reaction”), a methanol synthesis method, a C2-containing oxygen (ethanol, acetaldehyde) synthesis reaction and the like are well known. It is known that the FT reaction proceeds by an iron system catalyst containing iron, cobalt, ruthenium or the like, and the methanol synthesis reaction proceeds by a copper system catalyst and the C2-containing oxygen synthesis reaction proceeds by a rhodium system catalyst, and it is known that the catalytic ability of catalysts to be used in the synthesis of the hydrocarbons is strongly related to the dissociative adsorption ability of carbon monoxide (e.g., Homogeneous Catalysts and Heterogeneous Catalysts, edited by Hidai and Ichikawa, published by Maruzen, 1983).
On the other hand, low sulfur content gas oil is in demand in recent years from the viewpoint of atmospheric environmental protection, and it is considered that this tendency will become stronger in the future. Also, since crude oil is limited, development of an energy source that replaces the oil is in demand, and it is considered that this demand will become stronger in the future. As a technique that meets these demands, so-called GTL (gas to liquid) is known as a technique for synthesizing liquid fuels such as kerosine and gas oils from natural gas (main component: methane) which is considered to have recoverable reserves equivalent to crude oil on energy conversion basis. Since natural gas contains no sulfur components, or if contained, they are hydrogen sulfide (H2S), mercaptan (CH3SH) and the like which can be easily desulfurized, the thus obtained liquid fuels kerosine and gas oils and the like hardly contain sulfur components therein and they have advantages in that, e.g., they can be applied to high performance diesel fuel having a high cetane number, so that the GTL has been drawing attention more and more in recent years.
As a part of the GTL, a method for producing hydrocarbons from a synthesis gas by the FT reaction is being studied actively. In order to increase the yield of kerosine and gas oil fractions in producing hydrocarbons by the FT method, it is important to synthesize hydrocarbons equivalent to C10 to C16 hydrocarbons efficiently. In general, it is said that the distribution of carbon numbers of hydrocarbon products by the FT reaction follows the Shultz-Flory rule, and it is considered that, according to the Shultz-Flory rule, the chain propagation probability (α) value has a tendency to greatly decrease with increase in the reaction temperature, namely a tendency that the number of carbons of formed hydrocarbons is greatly reduced when the reaction temperature is increased. It seems that technical developments such as catalyst development and the like had been positively carried out formerly with the aim of selectively synthesizing hydrocarbons having a specified number of carbons by excluding the Shultz-Flory rule, but a technique which sufficiently resolved this problem has not been proposed yet. Rather, it is a recent idea that the yield of fractions which can be easily made into kerosine and gas oil fractions by hydro-cracking of wax components and the like is increased, not sticking to the exclusion of Shultz-Flory rule, and the wax components and the like are subjected to hydro-cracking to increase the yield of kerosine and gas oil fractions as the result. However, since the chain propagation probability (α) at the present time is around 0.85, it is one of the recent technical problems how to increase the value. Nevertheless, since the wax components become the majority of the formed hydrocarbons when the chain propagation probability (α) is increased too high, a problem on the process operation is generated instead so that, also from the viewpoint of general properties of the catalyst, it is considered that around 0.95 is the actual upper limit of the chain propagation probability (α).
Accordingly, in order to increase the yield of the kerosine and gas oil fractions further, it is necessary to consider formation of kerosine and gas oil fractions by forming lower olefin and carrying out its dimerization, trimerization and the like, in addition to the improvement of kerosine and gas oil fractions by forming wax components and carrying out the hydro-cracking. It is considered that such a still more improvement of the yield of kerosine and gas oil fractions can be achieved by carrying out the FT reaction which has high chain propagation probability (α), is excellent in the olefin selectivity in the formed lower hydrocarbon and is also excellent in the productivity of a liquid hydrocarbon fraction having a carbon number of 5 or more (hereinafter referred to as “C5+”).
Also, regarding the synthesis gas which is the material of the production of hydrocarbons by the FT method in the GTL process, the synthesis gas is mainly obtained by reforming a natural gas into a gas mixture comprising hydrogen and carbon monoxide as main components, by a reforming method such as autothermal reforming or steam reforming. However, since a water gas shift reaction of the following equation (II) occurs by this reforming in parallel with a reforming reaction of the following equation (I), carbon dioxide gas is inevitably contained in the thus obtained synthesis gas. In addition, since unused natural gas fields contain carbon dioxide gas in many cases, the use of such a carbon dioxide gas-containing natural gas as the material results in larger carbon dioxide gas content in the thus obtained synthesis gas.CH4+H2O=3H2+CO  (I)CO+H2O=H2+CO2  (II)
Also, as shown by the following equation (III), a liquid hydrocarbon is synthesized from the synthesis gas by the FT reaction, a tendency of obstructing synthesis of the hydrocarbon becomes strong when carbon dioxide gas is contained in the reaction system (Suzuki et al., Abstract of Papers, the 63rd Spring Annual Meeting of The Chemical Society of Japan, 3C432, 1992). Also, when the carbon dioxide gas content is increased, a hydrogen partial pressure in the reaction system is decreased in addition to the reaction inhibition of carbon dioxide gas, so that it becomes an undesirable situation for the FT reaction from this point, too.nCO+2nH2=(CH2)n+nH2O  (III)
Accordingly, it becomes essential for the conventional GTL process to incorporate a decarbondioxide step for removing carbon dioxide gas in the synthesis gas, between a step for producing a synthesis gas from a natural gas and a step for synthesizing a liquid hydrocarbon from the synthesis gas. Generally, amine absorption or pressure swing adsorption (PSA) is used in the decarbondioxide step, but such a decarbondioxide step is not desirable in any case, because it causes rise in construction cost and operation cost. When the decarbondioxide step can be simplified or omitted by enabling the FT reaction suitably in the coexistence of carbon dioxide gas, it can be greatly contributed to the reduction of production cost of the liquid hydrocarbon in the GTL process.
However, neither catalyst nor process, by which the FT reaction having high chain propagation probability (α) and excellent olefin selectivity and C5+ productivity and capable of sufficiently achieving further more improvement of the yield of kerosine and gas oil fractions can be carried out, has been proposed yet. Various catalysts for the FT reaction have been proposed, and a ruthenium catalyst comprising a manganese oxide carrier having provided thereon ruthenium, a ruthenium series catalyst in which a third component was added to the ruthenium catalyst, and the like have been proposed as catalysts for high selectivity for olefins (JP-B-3-70691, JP-B-3-70692, and the like). However, further more improvement of the yield of kerosine and gas oil fractions cannot be achieved sufficiently by the FT method using the ruthenium series catalysts. That is, since the ruthenium series catalysts are developed with the aim of using them in a fixed-bed system, the fixed-bed FT method using the ruthenium series catalysts has a problem in that the reaction cannot be carried out stably and smoothly, because not only insufficient chain propagation probability (α) and the like of the ruthenium series catalysts, but also the fixed-bed reaction system is apt to cause reduction of the catalytic activity when wax components are formed in a large amount, due to accumulation of the formed wax components to the active site of the catalyst and subsequent covering of the site, as well as its aptness to generate heat spots when the catalyst bed is topically overheated.
What is more, as described above, neither catalyst nor process, by which the FT reaction having high chain propagation probability (α) and excellent olefin selectivity and C5+ productivity and capable of sufficiently achieving further more improvement of the yield of kerosine and gas oil fractions can be carried out in the coexistence of carbon dioxide gas, has been proposed yet.